In an electronic homodyne receiver known in the prior art (as shown in FIG. 1(a)) a baseband signal provided using an RF carrier is down-converted from the RF carrier by mixing the received signal with a signal from a local oscillator (LO) that has the same frequency as the RF carrier (fRF). The mixing occurs in an RF mixer through the nonlinearities of electronic elements (such as diodes and transistors). The LO also uses electronic components as well as an electromechanical reference resonator such as a quartz crystal.
Today many RF communication links use an optical carrier to improve the bandwidth, reduce the loss and the overall cost by replacing bulky RF cables with thin optical fibers. In these links (also known as RF sub-carrier optical links) the data modulated RF carrier is up-converted to optical frequencies. The RF signals transported using optical frequencies (as the carrier) require conversion to electrical signals before processing. After conversion to electronic domain conventional electronic technology is used to process the RF signal. It is desirable to perform the required RF signal processing in optical domain in order to avoid the extra loss, cost and complexity associated with optical-to-electrical conversion. Moreover an all-optical communication system has the extra benefit of isolation from external electromagnetic radiation. As a result an all-optical link is not susceptible to electromagnetic noise and it is very secure (since it is not accessible wirelessly).
There is a need for an all-optical RF frequency converter that would eliminate the need to convert optical signals to electrical signals prior to processing those signals.